Techniques for producing composite lumps containing cBN are disclosed in, for example, Japanese Examined Patent Application, First Publication No. Sho 58-84106 and U.S. Pat. No. 2,947,617 in which a mixture of hexagonal boron nitride (hBN) and a catalyst is placed under pressure and temperature conditions that thermodynamically favor the stable presence of cBN.
Specifically, powder of hBN is mixed with a catalyst provided in the form of powder or lumps of an alkaline metal, an alkaline earth metal or a compound containing these elements. The resulting material is packed in a heater mainly made of graphite. The heater is then inserted in a capsule made of pyrophyllite. The material is then maintained under pressure and temperature conditions that thermodynamically favor the stable presence of cBN to produce composite lumps of cBN-containing material.
The cBN-containing composite lumps produced in this manner contain, aside from cBN, materials such as low-pressure phase boron nitride (i.e., remaining unreacted hBN, pBN, rBN, and recrystallized hBN), catalysts, graphite materials, and pyrophyllite. Thus, to produce cBN, cBN alone must be separated and recovered from the composite lumps.
Conventionally, the following method has been employed to separate and recover only cBN from the composite lumps.
The catalysts present in the composite lumps generally dissolve in water and thus can be separated by subjecting the crushed composite lumps to a process known as decantation. In this process, water is added to a mixture containing a substance of interest. After stirring, the solution is allowed to stand still to cause the substance to settle, and the supernatant is decanted. This process may be repeated more than one time.
The low-pressure phase boron nitride, pyrophyllite, and cBN contained in the composite lumps can be separated in a manner as described in, for example, Japanese Examined Patent Application, Second Publication No. Sho 49-27757. In this technique, composite lumps are crushed down to 5 mm or smaller in size. Sodium hydroxide is added to the crushed material along with a small amount of water, and the mixture is heated at about 300° C. to dissolve the low-pressure phase boron nitride in the sodium hydroxide solution.
The graphite material present in the composite lumps is separated and removed by dissolving the graphite in a mixture of sulfuric acid and nitric acid. Pyrophyllite, most of which in many cases exists as large chunks, may be removed manually by using tweezers or other tools.
As described above, the conventional technique for separating and recovering cBN from the composite lumps involves the use of large amounts of strongly acidic or strongly basic reagents in order to remove the low-pressure phase boron nitride, pyrophyllite, and graphite material and is therefore associated with considerable operational risks. The technique also requires significant amounts of labor and costly facilities to neutralize and properly process waste water. In particular, strongly basic reagents are used in substantial amounts and thus add to the operational risks and the amount of labor devoted to neutralizing the waste water. A further drawback is that the remaining low-pressure phase boron nitride in the composite lumps cannot be recycled because the low-pressure phase boron nitride is dissolved and discarded.